My naive opinion is, “leave package maintainers of those distros handle it”.
I don’t find it too problematic if the developer doesn’t provide binaries, if the code is reasonably easy to compile.
To be honest I didn’t even know ISVs were supposed to support older distributions. I always thought it was the distribution maintainers who would pick which release to include. For instance, if my software has v1 and v2, the maintainers would include v1 in Ubuntu 12 and v2 in Ubuntu 16.
ISV = Independent Software Vendor